A deceptively simple problem that applies to nearly all digital waveguide
models is that of delay-line interpolation. Integer delay lengths are not
sufficient for musical tuning of digital waveguide models at commonly used
sampling rates [31]. The simplest scheme which is typically tried
first is linear interpolation. However, poor results are obtained
in some cases (such electric guitars) because the pitch-dependent damping
caused by interpolation can be much larger than the desired damping in the
string model. In these cases, the interpolation filter becomes the
dominant source of damping, so that when the pitch happens to fall on an
integer delay-line length, the damping suddenly decreases, making the note
stand out as ``buzzy.''

Allpass interpolation is a nice choice for the nearly
lossless feedback loops commonly used in digital waveguide models,
because it does not suffer anyfrequency-dependent damping
[31]. However, allpass interpolation instead has
the problem that instantly switching from one delay to another (as in
a hammer-on or pull-off simulation in a string model) gives rise to a
transient artifact due to the recursive nature of the allpass
filter. Recently, Vesa Välimäki has developed a general transient
elimination scheme for recursive digital filters of arbitrary order
[64].

Another popular choice is Lagrange interpolation [33] which
is a special case of FIR filter interpolation; while the switching problem
does not arise since the interpolating filter is nonrecursive, there is
still a time-varying amplitude distortion at high frequencies. In fact,
first-order Lagrange interpolation is just linear interpolation, and higher
orders can be shown to give a maximally smooth frequency response at DC
(zero frequency), while the gain generally rolls off at high frequencies.
Allpass interpolation can be seen as trading off this frequency-dependent
amplitude distortion for additional frequency-dependent delay distortion
[16]. A comprehensive review of Lagrange interpolation appears
in [61].

Both allpass and FIR interpolation suffer from some delay distortion at
high frequencies due to having a nonlinear phase response at non-integer
desired delays. This distortion is normally inaudible, even in the
first-order case, causing mistuning or phase modulation only in the highest
partial overtones of a resonating string or tube.

Optimal interpolation can be approached via general-purpose
bandlimited interpolation techniques [56].
However, the expense is generally considered too high for widespread
usage at present. Both amplitude and delay distortions can be
eliminated over the entire band of human hearing using higher order
allpass or FIR interpolation filters in conjunction with some amount
of oversampling. A comprehensive review of delay-line interpolation
techniques is due to appear in [39].